We studied Na+ and Ca2+ currents in glial cells during the development of the corpus callosum in situ. Glioblasts and oligodendrocytes from frontal brain slices of postnatal day (P) 3 to P18 mice were identified based on morphological and ultrastructural features after characterization of the currents with the patch-clamp technique. Slices from P3-P8 mice contained predominantly glioblasts with immature morphological features. These cells showed Na+ and Ca2+ currents, but the population with these currents decreased between P3 and P8. Na+ currents were blocked in Na+-free bathing solution and in the presence of tetrodotoxin, Ca2+ currents were only observed when a high concentration of extracellular Ba2+ was present. The cells from the corpus callosum of P10 – P18 mice predominantly had morphological features of oligodendrocytes. In these cells, which in some cases were shown to form myelin, neither Na+ nor Ca2+ currents were detected. To compare these in situ results with those from the electrophysiologically and immunocytochemically well-characterized cultured glial cells, we determined the expression pattern of stage-specific antigens in the corpus callosum in situ. The first O4 antigen-positive glial precursors were observed at P1, the earliest stage examined. The oligodendrocytic antigens O7 and O10 appeared at P6 and P14, respectively, and prominent labelling with the corresponding markers was seen at P12 and P18, respectively. Despite the existence of numerous mature, O10-positive oligodendrocytes at P18, which expressed Ca2+ channels in vitro, we failed to detect Ca2+ currents in situ at this stage.